1. Field of the Invention
In general, the invention relates to vacuum contactors employing vacuum interrupters and in particular to the means employed to install and adjust the kickout spring used to open the vacuum contactor.
2. Description of the Prior Art
There are many designs of vacuum interrupters in existence. U.S. Pat. No. 4,002,867, issued Jan. 11, 1977 entitled "Vacuum Type Circuit Interrupters With a Condensing Shield at a Fixed Potential Relative to the Contact" is a representative example of such vacuum interrupters. An operating mechanism combined with one, two or three vacuum interrupters constitutes a vacuum contactor. In contradistinction to circuit breakers which are considered as principal protective devices during fault conditions in an electrical circuit and are designed for 20,000 to 50,000 operations, the vacuum contactor is used to start and stop various electric loads in response to signals generated by control devices such as push button switches, limit switches, and programmable controllers with the vacuum contactor being designed to have a lifetime of 2 to 3 million operations.
The main difference between vacuum contactors and conventional air break contactors is that the vacuum interrupters of the vacuum contactor break or interrupt the electric current inside a vacuum chamber instead of inside an air arc box. The vacuum chamber for the vacuum interrupter consists of a unit assembly of a sealed evacuated enclosure surrounding a fixed or stationary electrical contact and a moveable electrical contact. A portion of the moveable contact extends through a gas-tight metallic bellows which allows for the essentially linear motion of the moveable contact with respect to the stationary contact. The bellows is attached to the evacuated chamber by means of an end cap seal. Another end seal is provided for attaching the stationary contact to the enclosure. A ceramic sleeve or cylinder is provided to separate and electrically isolate the two contacts. The end seals are attached to the ends of the ceramic sleeve forming the evacuated chamber of the vacuum interrupter.
Because vacuum interrupters are normally closed by atmospheric pressure and an auxiliary contact spring, means must be provided to force the contacts into the open position which is the normal state for a deenergized contactor. The actual contact force holding the moveable and stationary contacts together inside each vacuum interrupter is the sum of the atmospheric force (atmospheric pressure times the mean area of the bellows) plus the force provided by the auxiliary contact spring and the mechanical spring force exerted by the bellows. This auxiliary contact spring force increases the total force sufficiently to sustain closure of the contacts during high short circuit currents that tend to blow the contacts apart. In the deenergized condition, there is no electrical energy available to provide the force necessary to separate the contacts. Instead, one or more mechanical springs provide this contact opening force. In practice this spring, called the kickout spring, exerts sufficient force to maintain the contacts in the open position in a deenergized contactor. To close the contacts of the vacuum interrupter on command, an electromagnet is provided that when energized, will pull the operating mechanism closed, overcoming the force of the kickout spring and closing the contacts of the vacuum interrupter.
One problem with typical vacuum contactors is the safe installation and adjustment of the kickout spring. The stored force in the spring, over 100 pounds in some cases, is potentially hazardous if the spring slips loose during installation or removal. Presently, control of the force in the spring during maintenance is somewhat haphazard. Existing designs compress the kickout spring between a moveable linkage and a plate secured to a base by fasteners such as screws or bolts. In order to remove the spring, the bolts or screws are loosened to relieve some of the force of the spring. However, the fasteners must usually be removed from the base before the force of the spring is reduced to an easily manageable level. Should the plate slip, the spring may cause injury to maintenance personnel. Similarly, in order to install the spring, it must be sufficiently compressed to allow the fasteners to engage the base. Again, should the fastener slip, the spring may cause injury to maintenance personnel. Thus, an adjusting means which can provide for the controllable increase or decrease in the force of the kickout spring would be advantageous.